This invention relates to a high speed gas chromatography system and particularly to such a system for improved rapid separation of polar compounds of an analyte mixture.
Gas chromatography (GC) is unsurpassed in its selectivity, sensitivity, and cost effectiveness. It is applicable for at least several hundred thousand compounds of low to moderate boiling point, including compounds in the C.sub.1 to C.sub.5 range. The process is also unique in its ability to obtain complete separation of complex mixtures of compounds.
In gas chromatography analysis the analyte mixture is separated into its components by eluting them from a column having a sorbent by means of a moving gas. In gas-liquid chromatography, which is a type in widespread use at present, the column comprises a nonvolatile liquid or solid sorbent coated as a thin layer on an inner support structure, generally the inside surface of a capillary tube. The moving gas phase, called the carrier gas, flows through the chromatography analytical column. The analyte partitions itself between the moving gas phase and the sorbent and moves through the column at a rate dependent upon the partition coefficients or solubilities of the analyte components. The analyte is introduced at the entrance end of the analytical column within the moving carrier gas stream. The components making up the analyte become separated along the column and elute at intervals characteristic of the properties of the analyte components. A detector, for example, a thermal conductivity detector or a flame ionization detector (FID) at the exit end of the analytical column responds to the presence of the analyte components. Upon combustion of the eluted components at the FID, charged species are formed in the flame. The flame characteristics are monitored through a biased ion detector which, along with associated signal processing equipment, produces a chromatogram which is a time versus detector signal output curve. The trace for complex mixtures includes numerous peaks of varying intensity. Since individual constituents of the analyte produce peaks at characteristic times and whose magnitude is a function of their concentration, much information is gained through an evaluation of a chromatogram.
While gas chromatography systems presently available perform satisfactorily, designers of such systems are continually attempting to optimize the capabilities of this separation procedure. Of particular interest is providing high speed gas chromatography for process stream control in industrial applications and in monitoring transient processes, for example, internal combustion engine exhaust gas compositions. The use of special inlet systems when combined with relatively short analytical columns operated at high carrier gas flow rates, has allowed separation of relatively simple mixtures on a time scale of a few seconds. However, some samples require much longer separation times because of the probability of co-eluting components. This probability is the result of the inevitable decrease in resolution when separation times are drastically reduced. To make high speed separation more practical, and to be able to apply fast gas chromatography techniques to a wider range of potential applications, it is necessary to enable adjustment of the selectivity of the system for specific sets of target compounds. Isolation of target compounds from other compounds minimizes potential chromatographic interferences and minimizes the separation time. By isolating the target compounds, the final separation step involves fewer compounds, and this can drastically reduce separation time.
In the analysis of some mixtures such as automotive exhausts and automotive fuels, chemical manufacturing process monitoring and the analysis of chlorinated hydrocarbon solvents in the environment, the particular compounds of interest are comprised of polar molecules. In many cases, other components of a sample such as so-called permanent gases, non-polar hydrocarbons and other classes of molecules are not of particular interest. Therefore, in some applications there is a need to quantify and separate only polar organic molecules. In conventional gas chromatography processes, other compounds such as non-polar organic molecules tend to co-elute with the target polar molecules of interest especially where the boiling points of the two groups of compounds are similar. The presence of co-elution renders it more difficult to separate the polar molecules since well defined chromatogram peaks are necessary for each molecule in the sample in order to successfully and accurately quantify it. As mentioned previously, the interests of obtaining rapid separation and definition and the ability to quantify sample constituents are conflicting. Accordingly, there is a need for a fast gas chromatography system for some applications in which target polar organic molecules can be rapidly separated from other types of molecules and separated to the degree that meaningful quantitive measurements can be obtained.
In accordance with this invention several embodiments of high speed gas chromatography system are described. Both embodiments utilize a tandem combination of analytical columns having polar and non-polar specific stationary phases. A combination of valves and pneumatic restrictors are used to isolate the polar compounds by means of venting and backflushing operations. The systems provide high speed separation and analysis of polar organic compounds with minimal sample loss, alteration or contamination attributable to the absence of valves in the sample flow path.
There are presently available gas chromatography systems used in industry for the separation of polar compounds which utilize a tandem combination of polar and non-polar analytical columns. The present practices are exemplified by ASTM procedure designation D 4815-89 which establishes a standard test method for evaluating certain alcohols and methyl tertiary butyl ether (MTBE) and gasoline by gas chromatography. In accordance with that procedure, however, a mechanical valve is implemented in the sample flow path which is used to direct the flow of sample and vent components which are not of interest. The device and the procedure established by the ASTM methodology possess a number of significant shortcomings. Of principal concern is the fact that by incorporating a mechanical valve in the sample flow stream, system dead volume is increased and the contamination of the sample by the valve is a constant concern. In addition, the detection limits of this system are extremely limited as well as its operational speed and flexibility. The gas chromatography systems in accordance with this invention provide significant improvements in undertaking this type of separation procedure.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.